Advanced oxidation processes are a type of chemical reactions that are often used to treat water, especially waste water, by removing contaminants (usually organic). Advanced oxidation is achieved by oxidizing the materials to be removed with hydroxyl radicals (hydroxide ions in their neutral form). However, there are a variety of different chemical processes used for wastewater treatment which fall under this umbrella, including some which employ ozone, hydrogen peroxide or UV light, or any combination thereof.

3. Reactions between hydroxyl radicals and small enough fragments result in either mineralization of the contaminant.

The variety of ways to achieve step one leads to the variety of specific methods used to perform advanced oxidation. A steady supply of ozone can be used to produce hydroxyl radicals, and combining UV light with a hydrogen peroxide molecule splits it into a pair of hydroxyl radicals. To perform advanced oxidation successfully, continuous use of one of these processes is required to produce a constant supply of ·OH.

Step two and three are more straightforward; once there is a sufficient supply of ·OH present in the water to be treated, it will continuously oxidize any available candidate molecules present in wastewater. These reactions produce only harmless stable molecules like water and carbon dioxide, or eventually precipitates, which are easy to remove non-chemically. The exact order in which reactions occur between hydroxyl radicals and all contaminants is not known, as there are a large number of different reactions that could occur, but all lead to the ultimate oxidation and mineralization of the contaminant.

Currently the primary drawback of advanced oxidation is the cost of sustained use. While an effective process, most methods of producing hydroxyl radicals require continuous supplies of expensive substances, and in some cases, large amounts of energy as well. By their very nature, advanced oxidation processes require a supply of hydroxyl radicals proportionate to the quantity of contaminants to be removed. This makes it extremely expensive to use advanced oxidation to remove contaminants from water with a high concentration of undesired substances; contemporary uses of advanced oxidation processes usually apply them after at least one stage of primary treatment using a different method.